The state-run China Daily newspaper, citing Beijing's weather officials, said melting snow made the air wet and caused heavy fog that "crippled traffic, delayed flights and created obstacles in the highways in many places of North China since Saturday."

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Forecasters predict winds from a cold front moving into central and east China Tuesday will "blow the fog away," the newspaper said.

Last month, when many official reports in Chinese state-run media referred to the air as being filled with "fog," the government acknowledged that the haze was due to smog.

While "fog alerts" are declared, "Ma Jun, director of the Institute of Public and Environment Affairs, said the hazes that have been smothering Beijing are really 'smog,'" the state-run China Daily reported in November.

Online monitoring from the U.S. Embassy in Beijing described the air quality as "hazardous" at numerous intervals throughout the day Sunday and Monday.

Chinese state media reports warned of traffic gridlock and poor visibility, but they did not mention health concerns.

"The air quality in Beijing had worsened as a result of the fog on Sunday and recovered a little on Monday," Xinhua said Monday, noting that China's National Meteorological Center's alert for the region was at the lowest level in the country's fog alert system.

The U.S. Embassy and Chinese officials use different systems to measure air pollution. The U.S. Embassy's air quality monitor includes smaller particles that are less than 2.5 micrometers in diameter. The U.S. Consulate General in Guangzhou, China, explains that these smaller particles "are believed to pose the largest health risks" and "are small enough to get into the lungs and even the blood stream."

The discrepancy has drawn criticism in online posts in China, which point to the different U.S. numbers and criticize the Chinese approach.

Chinese officials have criticized the American methodology, U.S. Ambassador Gary Locke said in an interview with CNN last month.

"They have disputes with the accuracy of our machine and the type of measurements that we're taking, but, as many other commentators have noted, it certainly is of greater and greater concern among the Chinese people," he said. The air quality, the environment and, in fact, their quality of life and the safety of food and water and air that they use every single day."

Experts have blamed the thick haze on rapid urbanization and industrialization.

Beijing, for instance, burned some 27 million tons of coal in 2010, according to state-run media. Despite efforts to limit the number cars with an auto-plate lottery, it's estimated that Beijing now has over 5 million cars, up from about 3.5 million in 2008.

Pollution is more acute because of the sheer size of the city's population (17 million) and the rapid speed of its economic growth, experts say.